An improved Na+-selective microelectrode for intracellular measurements in plant cells

The high background K+ concentration in plant cells is a problem for intracellular measurements of Na+ using ion-selective microelectrodes, The discrimination between Na+ and K+ of the microelectrode ionophore molecule limits the usefulness of this technique. A new Na+-selective microelectrode with an ionophore incorporating a tetramethoxyethyl ester derivative of p-t-butyl calix[4]arene has been developed, Microelectrodes made with this new sensor have superior selectivity for Na+ over K+ resulting in a lower limit of detection when compared with microelectrodes made using a commercially available ionophore (ETH227), Both types of microelectrodes were insensitive to changes in ionic strength and physiological ranges of pH, but only the calixarene-based electrodes showed no protein interference. To test the suitability of the calixarene-based microelectrodes for measurements in plants, they were used to measure Na+ in epidermal cells in the zone 10-20 mm from the root apex of barley (Hordeum vulgare L.). Seedlings were grown in a nutrient solution containing 200 mM NaCl for 1-6 d, The range of intracellular Na+ activity (a(Na)) measured varied from less than or equal to0.1 mM (limit of detection) to over 100 mFA, and these values increased significantly with time. The membrane potential (E-m) of these cells was variable, but the values became significantly more negative with time, although there was no significant correlation between E-m and a(Na). These intracellular measurements could not be separated into distinct populations that might be representative of subcellular compartments.